The present invention relates to a device for indicating the operating state of an electrical load and, more particularly, to a device for monitoring electric current in a cable connecting a load and a power source.
Many industrial, commercial and residential environments incorporate large numbers of electrical loads that are widely distributed geographically and often located in sites where access is difficult. Many these devices are small and draw very limited amounts of current, often only a small fraction of an amp. However, the operation of these loads, for example, small fan motors or lights can be important to maintaining a safe environment or the successful completion of a process that may involve costly or hazardous equipment or materials. Although desirable, monitoring the operation of these devices is complicated by their remoteness from the monitoring location. In addition, these devices are often controlled by a controller that is equally remote from the monitoring location. For example, while security or maintenance personnel may desire to monitor the operation of a building's lights from a central location, the lights of a commercial building are commonly controlled by switches, photo-detectors, or motion sensors located on the floor or in the room where the light is located. Likewise, an operator of an industrial process may desire to monitor the operation of a number of widely distributed devices, for example, the operation of a heater and a fan located in an air duct and controlled by a remotely located thermostatic sensor.
The state of the switch controlling the operation of a load can sometimes be signaled to a remotely located monitoring station, but the additional wiring and circuit complexity often makes monitoring the state of the switch impractical. For example, a second set of contacts in the switch that controls the load could be used to activate a relay signaling an open or closed connection between a power source and a load. However, the cost a switch with a second set of contacts, a relay, and wiring to connect the relay to a remote monitoring location is often prohibitively expensive.
Remote signaling of the operating status of an electrical device is commonly provided by a status indicator comprising a current sensor that is electromagnetically coupled to a cable supplying power to the monitored electrical device or load. Baron et al., U.S. Pat. No. 7,193,428, incorporated herein by reference, disclose a status indicator comprising a current sensor and a low threshold current switch. The status indicator includes a current transformer comprising an annular transformer core that encircles the power cable. Fluctuating current in the power cable produces a changing electromagnetic field around the cable which, in turn, induces a magnetic flux in the core of the current transformer. The magnetic flux in the core induces a voltage in a wire winding that encircles the cross-section of the core. Thus, the power cable is the primary winding and the wire winding is the secondary winding of the current transformer. The secondary winding is connected a rectifier and a capacitor that is selected to cause the circuit to resonate at the expected frequency of the alternating current induced in the secondary winding, typically 50-60 Hz. The output of the resonant circuit, comprising the secondary winding and the resonating capacitor, is the input to a voltage multiplier. When a current is present in the power cable, a voltage is induced in the secondary winding which is multiplied in the voltage multiplier causing a pair of transistors of a current switch to conduct shorting the output terminals of the status indicator. When there is no current in the power cable, a voltage is not induced in the secondary winding and the switch transistors do not conduct, preventing conduction between the switch output terminals. The low threshold current switch is capable of detecting currents less than 0.15 amps making the status indicator particularly useful for loads that draw very limited currents.
The low threshold current switch can be implemented with either a solid core current transformer, as illustrated in U.S. Pat. No. 7,193,428 or a split core current transformer. Passing the power cable through the central aperture of a solid core transformer requires that the power cable be disconnected so that the end of the cable can be inserted into the aperture. This can be particularly difficult when a status indicator is to be retrofitted to an existing circuit and the most desirable location of the device is between distantly located termini of the power cable. Cota, U.S. Pat. No. 5,502,374, incorporated herein by reference, discloses a split core current transformer that enables engagement of a power cable without disconnecting the cable. The hinged case enables the halves of the toroidal core to pivoted apart. The cable can pass between the open ends of the core portions and then be secured in the central aperture of the core by pivoting the portions of the core back together. While a split core sensing transformer facilitates installation, particularly where it is difficult to pass a disconnected end of the cable through the core's aperture, the current transformer is quite large making it difficult to locate the device in the small electrical enclosures and spaces that often typify installations incorporating loads having limited current draws.
What is desired, therefore, is a very compact device for detecting and indicating the status of current flowing in an electrical conductor.